The invention relates to a heater element, a heater and a process for producing the heater element and heater. In particular, the heater comprises an electrically conductive fabric layer laminated between two layers of glass fiber-reinforced thermoplastic films. The heater further comprises bus bars and electrical leads, and it is produced by consolidating the layers of film and fabrics into a single sheet heater. The heater can be produced having variable geometry, electrical supply voltage and power. The heater of the invention is more durable than prior art heaters because it is able to withstand more mechanical, chemical, ultraviolet radiation and other environmental stresses than prior art heaters.
Laminate or film heater of the prior art have been made using metal foil, wires and electrically conductive fabrics laminated together using resins in between the layers to bond the integral layers together. Heaters manufactured from wire and foil have been used in industry for some time. In particular, such wire and foil heaters when used, for example, in the aerospace industry for deicing structures such as airplane wings and jet engine inlets, suffer from fatigue failure while in use, which considerably shortens their life when compared to fabrics. In addition, foil heaters, in particular, are expensive to produce and lack flexibility.
Because of their flexibility, light weight and even heat distribution characteristics, prior art laminate fabric heaters have been preferred in the art for many applications over metal foil heaters. In the aerospace industry, for example, fabric heaters are used for deicing structures such as airplane wings, jet engine inlets and antenna dishes, in the buildings industry for heating solid structures such as floors, countertops, pipes and tanks; in the food industry for heating food receptacles and in shipping industry and marine structures for preventing biofouling. See for example, U.S. Pat. Nos. 5,344,696; 5,925,275; 5,932,124; 5,942,140; 5,954,977; 5,966,501; 5,981,911.
Laminate fabric heaters of the prior art have been made with woven or non-oven fabrics containing fibers, which are electrically conductive fibers such as carbon fibers, and non-conductive fibers such as polyester. Non-conductive fibers for use in heaters are usually coated with a metal so that they can conduct the current via the metal coat, or the fibers are dispersed in a resin containing conductive particles, such as carbon black or iron particles. Conductive fibers can also be coated with a metal to improve their conductive properties.
Carbon fibers consolidated into a random, non-woven fabric or veil have been used in the art for shielding against electromagnetic interference. Conductive fabrics used in heaters for deicing and anti-icing aerospace structures are disclosed in, for example, U.S. Pat. No. 5,344,969 to Hastings et al. discloses an integrally bonded laminate that is used to thermally control a surface of an aircraft to which the laminate is bonded. The patent describes that the use of fabrics have numerous advantages over prior methods for deicing and heating airplane wings; for example, the conductive fiber is of low weight, and or permits nominal intrusion in terms of aerodynamics; it is easy to handle compared to wire and foil, and most notably, it allows the even distribution of heat. These factors contribute to a more efficient use of energy. Deicing and anti-icing aircraft applications necessitate an extreme in terms of product requirements. Because aircraft operate on very limited electrical resources and extreme atmospheric conditions, a system must be efficient as well as robust to provide protection. The patent also discloses, however, that the laminated fabric heater is manufactured using adhesive resins to bond the laminating layers together.
U.S. Pat. No. 4,534,886, to Kraus et al., discloses an electrically conductive web composed of a non-woven sheet of conductive fibers and non-conductive fibers. The sheet is saturated with a dispersion containing conductive particles and is then dried. The Kraus et al. heater element is used primarily in heating pads. The patent also discloses that the fabric heater layers are laminated together using an epoxy resin.
U.S. Pat. No. 5,925,275 to Lawson discloses an electrically conductive composite heating assembly. The assembly has an electrically conductive non-woven fiber layer laminated between layers of fiberglass and other dielectric material. The assembly further has an abrasion resistant outer layer. The heater element is used on aerospace structures as an ice protection system to withstand the repeated mechanical stress and thermal cycles encountered in extremely harsh aerospace environments. This patent also discloses that the fabric heater is manufactured using adhesive resins to bond the layers of the heater assembly.
Conductive fabric heaters disclosed in UK Patent Application No. 2,285,729 to Gerrard, are manufactured by baking a woven polymeric fabric to extreme temperatures in a multi-step process. The resultant fabric heater is limited by flexibility in resistance goals and furthermore lack variability of its Temperature Coefficients of Resistance. For example, the patent discloses that the fabric heaters are primary useful for low voltage source operation.
While laminated fabric heaters made using adhesive resins have been used with some success in the art, a disadvantage of using these type of heaters is due to the adhesive resins used. Adhesive resins used in making or bonding such laminated heaters must be cured and the process becomes time consuming and dangerous due to the toxicity of the materials involved. In addition, while adhesive resins are widely used to make laminated fabric heaters, the heat output from these type of heaters over a period of time dries the resin, leading to cracking of this layer and ultimately, the heater delaminates and loses function. Therefore, the art always seeks to develop new heaters or to improve the existing heaters. while adhesive resins are widely used to make laminated fabric heaters, the heat output from these type of heaters over a period of time dries the resin, leading to cracking of this layer and ultimate, the heater delaminates and loses function. Therefore, the art always seeks to develop new heaters or to improve the existing heaters.
In addition, heater designs as mentioned in Kraus, Hastings and Lawson suffer from expensive production methods and low operating temperatures. Machine setup is difficult to modify for different applications especially due to the use of epoxy and other resins, which have problems difficult to overcome; such as cost, shelf life, operating temperature and chemical limitations, long curing cycles and toxicity precautions. The addition of carbon black particles, polyaramid fibers (1), conductive adhesives (3), and multistage layer processing (2) contribute to the complexity and therefore the heater cost while limiting service temperature, suitability for complex designs, and high volumexe2x80x94low cost production.
The invention is directed to a laminated fabric heater element, a heater and a process for manufacturing the heater element and heater. The heater of the invention has many advantages over prior art heaters in that it is thin, flexible, produces more uniform temperature, has high fatigue life, and can be mass produced at less costs. In addition, the heaters of the invention can be operated at voltages ranging from millivolts to about 600 volts from either alternating current or direct current power supplies.
Specifically, the heater element comprises a consolidated electrically conductive fabric layer, two bus bars, and two thermoplastic layers; wherein each bus bar is contacting opposing edges of the conductive fabric layer and the consolidated electrically conductive fabric layer and the bus bars are sandwiched between the thermoplastic layers forming a single sheet.
The electrically conductive fabric layer of the laminated fabric heater of the invention may be selected from a variety of conductive fibers. However, in a preferred embodiment of the invention, the electrically conductive fabric layer comprises nickel-coated carbon fibers.
The laminated fabric heater element can further be attached to electrical leads at bus bars to form the heater. The bus bars of the laminated heater can be made of various material such as copper, brass or silver foils. In a preferred embodiment, however, the bus bars are made of copper foils.
In another embodiment, the laminated fabric heater of the invention can further comprises a glass veil disposed on the outer surfaces of the thermoplastic layers for additional reinforcement depending on the requirements for the heater application before the heater is formed into a single sheet.
In the heater of the invention, the thermoplastic films can be obtained from commercially available sources. While any thermoplastic film can be used, the heaters of the invention are preferably manufactured with polyetherimide, polyetheretheketone, polyethersulfone, polysulfone polyvinylidine fluoride, acetobutylstyrene, polyphenylene oxide and polyamide.
The laminated fabric heater can further comprises cuts perpendicular to and through at least one of the bus bars in a zig-zag pattern for creating a circuit and to increase the resistance of the heater to a desired value depending on the application.
In another embodiment, the laminated fabric heater of the invention further comprises an outer layer of thermoplastic or silicon rubber for increasing the dielectric strength of the heater.
The process for making the laminated fabric heater of the invention is as follows: A first thermoplastic layer on a surface where the heater is to be assembled. A layer of the electrically conductive fabric is disposed on the first thermoplastic layer. Bus bars, preferably made of copper foil are disposed on opposing edges of the electrically conductive fabric layer so that the bus bars are in contact with the electrically conductive fabric layer and are parallel to one another. Once the bus bars are in contact with the conductive fabric, they can be attached to the conductive fabric by piercing a hole through the bus bar and fabric using a piercing rivetor apparatus. The action of piercing causes the metal displaced to form a hole to curl and flatten under the fabric, thereby securing the bus bars to the fabric. Thereafter, a second thermoplastic layer is disposed on the electrically conductive fabric layer and bus bars to form a heater assembly. Once the heater layers are assembled, the heater assembly is heated at suitable temperatures to a set thickness to consolidate the conductive fabric layer sandwiched in the thermoplastic film layers, thereby forming a single sheet heater. After consolidation of the layers, and specially of the conductive fabric layer, the heater is transferred to a cooling chamber to quench the heater at its maximum consolidation state. A glass fiber reinforcement layer can be disposed on the outer surfaces of the thermoplastic layers prior to consolidation and depending on the heater output requirement.
While the process described above can be performed at a small scale to produce a small number of small heaters using a hydraulic press, the process can be adapted for manufacturing heater elements and heaters in high volume using a roller laminating apparatuses. In roller lamination, the heater element is produced in a single long sheet of indefinite length and width, which length and width are only limited by the length and width of the starting materials and machinery used. The heater element made through roller lamination can be stored in rolls, and heaters can be made from segments of the heater element as required. In this embodiment of the invention, the process comprises combining a layer of electrically conductive fabric from a roll supply with two metal foil bus bars, wherein the bus bars are positioned parallel to one another at opposing edges of and contacting the conductive fabric in the direction of the roll. The bus bars are secured to the conductive fabric by making a hole in the conductive fabric and bus bar by piercing both components in a piercing rivetor apparatus as described above. Once the bus bars are secured to the fabric, the conductive fabric layer containing bus bars are drawn between tow layers of thermoplastic films forming a sandwich type structure assembly. The heater assembly sandwich is then fed through a pinch roller, which had been preheated at a predetermined temperature and set at a predetermined pressure to cause gelling of the thermoplastic layers. The gelling of the thermoplastic layer causes some of the thermoplastic to flow through the conductive fabric, fusing the films and consolidating the conductive fibers into a single sheet heater element. Once consolidation occurs, the resultant single sheet fabric heater is drawn over a cooling chamber so that maximal consolidation of the layers is maintained. Individual heaters can be made by cutting a section from the heater sheet roll with a tooling die or a water jet cutter, attaching electrical leads by ultrasonic welding and laminating it once more with a layer of thermoplastic, thereby maintaining the gap cut by the die or water jet and providing a final dielectric layer.